python3: help with pickle

[Chris Rebert]

On Thu, Jul 15, 2010 at 3:34 AM, Alan <alanwilter at gmail.com> wrote:
> Hi there,
> This is more an exercise to myself to understand python3. I took a code I
> wrote (acpype) and I am trying to make it compatible with either python 3 or
> 2.
> I am trying to make a pickle file compatible with either python 3 and 2 as I
> believe it should be possible.
> I've looked at http://docs.python.org/py3k/library/pickle.html but still,
> when I create a pickle file for an object with python 2.7 and then I try to
> load it in python3, I got this:
> Python 3.1.2 (r312:79147, Jul  7 2010, 10:55:24)
> [GCC 4.2.1 (Apple Inc. build 5659)] on darwin
> Type "help", "copyright", "credits" or "license" for more information.
>>>> from acpype import * # this acpype package contains needed class ACTopol
>>>> import pickle
>>>> o = pickle.load(open('AAA.acpype/AAA.pkl','rb'))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "/sw/lib/python3.1/pickle.py", line 1365, in load
>     encoding=encoding, errors=errors).load()
> TypeError: ('__init__() takes at least 2 positional arguments (1 given)',
> <class 'acpype.ACTopol'>, ())
>>>>
> Now trying the contrary, pickle file is created with python3, I got this
> when trying to load it with python 2.7:
> Python 2.7 (r27:82500, Jul  7 2010, 10:48:15)
> [GCC 4.2.1 (Apple Inc. build 5659)] on darwin
> Type "help", "copyright", "credits" or "license" for more information.
>>>> from acpype import *
>>>> import pickle
>>>> o = pickle.load(open('AAA.pkl','rb'))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "/sw/lib/python2.7/pickle.py", line 1378, in load
>     return Unpickler(file).load()
>   File "/sw/lib/python2.7/pickle.py", line 858, in load
>     dispatch[key](self)
>   File "/sw/lib/python2.7/pickle.py", line 1083, in load_newobj
>     obj = cls.__new__(cls, *args)
> AttributeError: class ACTopol has no attribute '__new__'
> Apart that, everything else seems to work. Just one note more: when loading
> the pickle file 2.7 in python 2.7, type(o) is <type 'instance'>, while
> pickle 3 in python 3, type(o) is <class 'acpype.ACTopol'>

Your AbstractTopol class needs to inherit from class `object` so that
it (and its descendant ACTopol) become new-style classes. Old-style
classes don't exist in Python 3, and I think this is at least partly
the cause of your problem.

Cheers,
Chris
--
http://blog.rebertia.com